Hillary Clinton warned Somali pirates that the US will track down and freeze the money they are using to fund banditry on the Indian Ocean.

The US secretary of state promised to use “21st century solutions” for a 17th century crime, on another day of drama off the Somali coast. The French navy detained 11 Somali pirates trying to seize a Liberian-flagged merchant ship, just hours after a US cargo vessel came under rocket fire and had to take evasive action to shake off its attackers.

The US is planning to convene an anti-piracy task force to co-ordinate international naval action in the area, and to push for diplomatic efforts in Somalia, where the international community is urging help from the largely powerful transitional central government and from local warlords.

“We have noticed that the pirates are buying more and more sophisticated equipment, they're buying faster and more capable vessels, they are clearly using their ransom money for their benefit – both personally and on behalf of their piracy,” Mrs Clinton said. “We think we can begin to try and track and prevent that from happening. We may be dealing with a 17th-century crime, but we need to bring 21st-century solutions to bear. These pirates are criminals, they are armed gangs on the sea. And those plotting attacks must be stopped.”

The Obama administration plans to send an envoy to a conference on piracy in Brussels next week. The long-term solution will be development and restoring the rule of law in the country, which has been without an effective central government since 1991, Mrs Clinton said.

The United Nations authorised a “hot pursuit” policy last year that allows navies to go after pirate bases on the shore, but the US military has not so far adopted that as a policy.

There has been no let up in pirate activity since last weekend’s dramatic rescue of the American freighter captain, Richard Phillips, when US snipers killed three Somali pirates to end a five-day hostage drama. Brigands have seized four vessels and more than 75 hostages since Sunday.

A new incident late on Tuesday delayed the reunion of Captain Phillips with his crew, because the US naval destroyer Bainbridge, which was carrying him to Mombasa in Kenya, had to be diverted to respond to a distress call from another ship. The Liberty Sun, carrying food aid, took evasive action after being fired upon by pirates. And yesterday, French officials said they had detained 11 pirates trying to seize a Liberian ship. The French frigate Nivose captured a pirate gang's mother ship, which was carrying two small assault boats, about 560 miles east of the Kenyan port of Mombasa on Tuesday, the French Defence Ministry said in Paris.

The Nivose, deployed to prevent attacks in the gangs' widening hunting grounds, tracked the pirates after its helicopter thwarted an attack on the Liberian-flagged Safmarine Asia. France’s frigate is in the region as part of “Operation Atalanta”, the European Union’s anti-piracy mission that also involves German, Spanish, French and Italian forces.